Most nature waters contain boron in a very low concentration, for example the boron concentration of a drinking water is usually much lower than 5 ppm. However, the boron concentration in an industrial processing water may be quite different. In the primary coolant of pressured water reactor (PWR) of nuclear power plant, the range of boron concentration may vary from 2000 ppm to a few ppm in order to control the reactivity of reactor. In addition, a large volume of slightly radioactive wastewater containing boron primarily as boric acid may be annually generated from this type of nuclear power plant. The wastewater is required for treatment. Therefore, an efficient process or a method used for boron separation is essential for controlling the industrial process and for the wastewater treatment.
Boron compounds like boric acid are widely used as raw material for industries particularly in the areas of glass, ceramics and enamels. Boric acid is also used as a starting chemical for production of borate salts, boron phosphate, fluoroborate, borate esters and metal alloys. A cost-efficient process for separation or recovery of boron is required for these industries.
Boron mostly presents as a weakly dissociated anion in normal aqueous solution. The distribution of boron species depends on the pH of solution and the concentration of boron (CRC, 2001). In a low concentration of solution with a pH of around 5, most of the boron exits as boric acid, H3BO3, which is an uncharged species. At an increased pH of up to 10 the anion form of boron, H2BO3−, becomes dominant. In a high concentration of solution such as in the primary coolant of pressured water reactor (PWR), boron may be distributed in six species such as boric acid, tetrahydroxyborate (B(OH)4−), septahydroxydiborate (B2(OH)7−), decahydroxytriboroate (B3(OH)10−), tetradecahydroxytetraborate (B4(OH)142−), and octadecahydroxypentaborate (B5(OH)183−) (Sperssard, 1970). A general equilibria among these species can be expressed as follows:xH3BO3+yOH−Bx(OH)3x−y  (1)The polymerisation of boric acid easily takes place in a high concentration of boron solution (B>1000 ppm).
There are serious challenges for the separation and recovery of boron from aqueous solution, because boron mostly presents as non-dissociated boric acid in neutral or weakly basic solutions. The rejection of boron in a reverse osmosis system is low (between 40-60%) under normal operating conditions, although an increase in the rejection may be achieved at pH of 9.5 or above (Prates et al., 2000). The non-dissociated boron cannot be removed by conventional ion-exchange technique since ion-exchange resin can only exchange ionised substances. Electrically driven membrane techniques such as electrolysis or electrodialysis are not suitable for the separation of boron because uncharged species cannot be easily migrated in an electrical field (Melnik et al., 1999).
There is an approach to remove boron using boron-selective resins (chelating resins) with diols as the complexing agents of boron (Nadav, 1999; Simonnot et al., 2000; Wilcox et al., 2000). However, it is usually expensive and requires a complicated regeneration procedure. Moreover, the recovery of boron requires a selective separation of boron from other anions such as chloride, nitrate and sulfates in aqueous solution.
A technique so-called electrodeionisation (EDI), which combined electrodialysis and ion-exchange, was used to remove ionisable species from aqueous solution by Kollsman et al., (U.S. Pat. Nos. 2,689,826 and 2,815,320). Improved EDI systems were disclosed and commercialised by Giuffrida et al., (U.S. Pat. Nos. 4,925,541 and 4,931,160), Ganizi et al., (U.S. Pat. Nos. 5,308,466 and 5,316,637), and Springthorpe et al., (U.S. Pat. No. 5,868,915) for the purification of waters. The most of electrodeionisation systems and apparatus were used for water purification and removing relatively low concentration of ionised contaminants from water. Although it has been reported that EDI can remove some weakly dissociated anions like carbonates, it is still a challenge for the EDI to remove trace boric acid and silica from aqueous solution. Moreover, the EDI has not been used for the purposes of separation, recovery or purification of weakly ionisable compounds like boric acid.